1. Field of the Invention
This invention relates to a method and apparatus for the measurement of a physical property of a fluid that is dependent upon a physical characteristic of at least one functional group and is related to the quantity of that functional group in the fluid. More particularly, this invention relates to the measurement of the heating value of a fuel gas at-line and in real time. Even more particularly, this invention relates to a method and apparatus for measuring the heating value of a combustible gaseous fuel mixture, including functional groups and molecules, using near infrared absorption spectroscopy.
2. Description of Related Art
Historically, the heat energy content of a combustible fluid was determined by burning precisely defined amounts of the fluid, e.g. natural gas, to determine the amount of energy produced from the combustion. Other methods determined the concentration of each whole combustible compound in the mixture, defining the energy content for each whole combustible compound, and summing them to yield the heat energy content of the entire mixture.
The heat energy content of natural gas flowing through a pipeline, which natural gas typically contains methane, ethane, propane and higher alkane hydrocarbons, frequently fluctuates, even over relatively short periods of time. Conventional methods of measurement generally require bypass flow-lines or fluid extraction to provide gas samples which are then taken to a lab and burned. The temperature of the flame is then measured. It is difficult to both continuously and accurately measure the energy content of natural gas in pipelines, and the lack of any convenient means for making such continuous and accurate measurements may result in improper charges during the course of a day to the disadvantage of both buyers and sellers.
Commercially, there are no known products capable of accurately determining the heating value of a fuel gas without removing a gas sample and reducing the sample pressure for analysis. Available sensors are primarily comprised of calorimeters and gas chromatographs (GCs). However, such devices, in addition to requiring the removal of samples from pipelines, have slow response times, and have high initial and maintenance costs.
One technique for addressing the need for both continuous and accurate measurement of the heat energy content of combustible gaseous fluid mixtures employs infrared spectroscopy in which infrared radiation causes groups of atoms of organic compounds to vibrate about their covalent bonds. Because of the vibrations, the groups of atoms absorb a quantified amount of infrared energy in particular regions of the spectrum. U.S. Pat. No. 4,594,510 to Brown et al. teaches a heat energy measuring system which directs radiation through a sample of a combustible fluid and detects the absorbance of at least one combustible component of the combustible fluid at a selected spectral line, where there is at least one spectral line for each combustible component to be examined in the fluid. The system also combines at least one heat energy proportionality factor with the absorbance at each spectral line and sums these combinations to determine the heat energy of the fluid. Calibration for specific hydrocarbon species is achieved with an on-board system of individual cells of gases from which calibration matrices are calculated. This method of calibration disadvantageously adds a significant amount of time and complexity to the system.
U.S. Pat. No. 5,822,058 to Adler-Golden et al. teaches the use of absorption spectroscopy to derive the heat of combustion of a combustible mixture. The absorption spectrum is measured by utilizing a light source, light dispersing device, and a detector. However, the heat of combustion is not measured directly. Rather, the mixture composition is first determined from which the heat of combustion is derived by relating individual hydrocarbon heats of combustion. The '058 patent further teaches the use of absorption spectra of natural gas over the wavelength range of about 700-1000 nm. The detector is a silicon detector which cannot be used to detect wavelengths higher than about 1000 nm. Light source degradation is addressed by allowing a second fiber-optic cable from the light source to be directed straight to the spectrometer and then to the detector. This is able to be accomplished because a 2D detector is used. This larger detector has more surface area, thereby allowing for a second spectrum to be analyzed.
U.S. Pat. No. 6,555,820 B1 to Tacke et al. teaches a photometric device and method for determining the gross calorific value of natural gas having a radiation source that produces a measuring beam and a modulation unit to modulate a measuring beam. A test cell with a test gas and a receiver for the beam are arranged successively in the path of the measuring beam. The measuring signals of the receiver are supplied to an evaluation unit that includes at least one signal amplifier for amplification of the signals. The gross calorific value of the gas is indicated by the sum of the amplified measuring signals produced in a computing machine. However, no means are provided for tracking the fluctuations or degradation of the light source spectrum, which fluctuations or degradation may constitute a large source of error when calculating gas energy content.